What You Need To Know About Steel Corrosion: The Basics

No matter what kind of project you are working on, protecting against steel corrosion will be something of primary concern at one point or another. For all its strength and versatility, steel will eventually rust. Luckily, this inevitability is something that is easily reconciled with steel surface treatment. If you have any questions concerning protecting your steel against corrosion or any other interference from malicious agents, please get in touch here. Here are some of the basics about steel corrosion you may find useful too.

 

What Is Corrosion?

Steel corrosion can be defined as deterioration due to electrochemical reactions with the metal and its environment. Specifically for steel, rusting occurs when the iron in the metal oxidises. In most cases, in order for corrosion to occur, both oxygen and water must be present.

The process of steel corrosion can be broken down into stages:

• Anodic areas on the surface are attacked and ferrous ions go into solution.

• Electrons are then released, they travel through the metallic structure to cathodic sites on the surface. Here they combine with oxygen and water to form hydroxyl ions.

• These react with the ferrous ions from the anode to produce ferrous hydroxide.

• This further oxidises to produce hydrated ferric oxide which is known as ‘red rust’.

This chemical reaction can be summarised like this:

Fe + 3O2 + 2H2O = 2Fe2O3H2O or (Steel) + (Oxygen) + (Water) = Hydrated ferric oxide (Rust) 

Corrosion is most common when water is present (think humid environments) and the process is accelerated by salty water. Steel corrosion can also occur without the presence of water where the iron oxidises in the atmosphere. This is called dry corrosion and can be accelerated by higher temperatures.

What Are The Types of Corrosion?

There are many types of corrosion. Here are a few of the most common:

• Uniform attack corrosion – Also known as general attack corrosion, this results in the deterioration of the entire exposed surface of the steel. It is the most common kind of decay.

• Galvanic corrosion – Galvanic corrosion, or dissimilar metal corrosion, occurs when steel comes into contact with another metal in a corrosive electrolyte.

• Pitting corrosion – Pitting corrosion occurs when localised areas of steel lose their protective layer. This area becomes anodic, while part of the remaining metal becomes cathodic, producing a localised galvanic reaction.

• Crevice corrosion – Similar to pitting, crevice corrosion is localised, generally in a stagnant microenvironment where acidic conditions, or a depletion of oxygen in a crevice can lead to corrosion.

• Intergranular corrosion – Intergranular corrosion occurs along the boundaries of metal crystallites or grains.

 steel corrosion

Types of Steel Surface Treatment

Surface treatment is the most effective way to protect against steel corrosion. Here are some of the most common you will see steel fabricators use:

Hot dipping – This galvanisation method can be used on steel of all shapes and sizes and involves immersing the steel in a bath of molten zinc at temperatures up to 450°C. Steel galvanised in this way is highly protected against corrosion as well as extreme weather conditions and this is a particularly popular approach for pipe related applications.

Zinc phosphate priming – Steel surfaces can be painted with specially formulated primers in order to improve corrosion resistance as well as boost their visual appeal, with one of the most popular applications being zinc phosphate priming.

Chemical coating – This brilliant technique uses electrostatic or compressed air to apply a specially formulated powdered material to the steel surface, following which it is melted to form a smooth protective film. Steel treated in this way is not only protected against corrosion and UV damage, it is also highly resistant to peeling, scratching and cracking.

Zinc spray metallising – While it doesn’t provide quite the same protection as hot dip galvanising, this technique is highly effective against corrosion and is popular for its smooth finish. Because this is a ‘cold process’ there is no risk of distorting the metal, making it ideal for use on ornate metal components such as railings and fences.

What Affects Corrosion Rate?

Corrosion rate is the speed at which the steel deteriorates in a specific environment. There are two main constituents that affect the speed at which the steel will deteriorate: how long the steel is exposed to moisture, and exposure to other malicious agents in the atmosphere. As environments that steel structures exist in differ greatly, there is no set corrosion rate as such.

Other malicious agents that will contribute to corrosion include sulphates (which you will often find in industrial areas and become quite corrosive when combined with water), chlorides, salt, bleach and acid.

Characteristics which will affect the corrosion rate include the surface area of steel that is exposed, how long it is exposed for, and the density of the steel and heat.

What Is Stainless Steel?

Stainless steel has an extremely high resistance to corrosion due to its substantial chromium content (above 10.5%). This resistance to corrosion is due a sort of naturally-occurring ‘surface treatment’ (as it were) that is inherent to the steel. An ultra thin chromium-rich oxide film offers very efficient anti-corrosive properties.

Low alloyed grade stainless steel resist corrosion in atmospheric conditions, while highly alloyed grades can resist corrosion in most acids, alkaline solutions, and chloride bearing environments, even at elevated temperatures and pressures.

You can read about how stainless steel was invented here.

If you have a project in need of steel, whether it’s supply, fabrication, drafting, or installationtalk to the experts at Steel Fabrication Services. Our dedicated team of professionals will ensure that your project goes according to plan right down to the smallest detail. Give us a call today!

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